In the development of radio communication systems, such as mobile communication systems (like for example GSM (Global System for Mobile Communication), GPRS (General Packet Radio Service), UMTS (Universal Mobile Telecommunication System) or the like), efforts are made for an evolution of the radio access part thereof. In this regard, the evolution of radio access networks (like for example the GSM EDGE radio access network (GERAN) and the Universal Terrestrial Radio Access Network (UTRAN) or the like) is currently addressed in research and development as well as in standardization. Accordingly, such improved radio access networks are sometimes denoted as evolved radio access networks (like for example the Evolved Universal Terrestrial Radio Access Network (E-UTRAN)) or as being part of a long-term evolution (LTE). Although such denominations primarily stem from 3GPP (Third Generation Partnership Project) terminology, the usage thereof hereinafter is not intended to limit the respective description to 3GPP technology, but is rather intended to generally refer to any kind of radio access evolution irrespective of the specific underlying system architecture. Another example for an applicable broadband access system may for example be IEEE 802.16 also known as WiMAX (Worldwide Interoperability for Microwave Access).
In the following, for the sake of intelligibility, LTE (Long-Term Evolution according to 3GPP terminology) is taken as a non-limiting example for a broadband radio access network being applicable in the context of the present invention and its embodiments. However, it is to be noted that any kind of radio access network may likewise be applicable, as long as it exhibits comparable features and characteristics as described hereinafter.
In the development of cellular systems in general, and access networks in particular, relaying has been proposed as one concept. In relaying, a user equipment or terminal (UE) is not directly connected with an access node such as a radio base station (e.g. denoted as eNodeB or eNB) of a radio access network (RAN), but via a relay node (RN). FIG. 1 shows a schematic diagram of a typical deployment scenario of a relay-enhanced access network with radio-relayed extensions.
Relaying by way of relay stations (RSs) or relay nodes (RNs) have been proposed for the purposes of extending the coverage of cellular systems, providing a high-bit-rate coverage in high shadowing environments, reducing average radio-transmission power at the user equipment so as to prolong battery lifetime, enhancing cell capacity and effective throughput, e.g. increasing cell-edge capacity and balancing cell load, and enhancing overall performance and deployment cost of radio access networks.
A requirement for the deployment of relay-based extensions (for example in LTE-Advanced) is backward compatibility with current access networks (in case of LTE, for example with LTE-Release 8). This is especially important from the UE side, as it will allow users to benefit from relaying with Release 8 terminals. That is, LTE-Release 8 terminals and LTE-Advanced terminals should work equally well in LTE-Release 8 and LTE-Advanced networks. At the network side, software and even hardware updates between standard releases may be possible, but preferably they should be as small as possible. Hence, from the UE side, the serving network node should function in exactly the same way as a Release 8 access node (i.e. an eNB).
When reference is made to relaying, especially in the context of relay-enhanced access networks such as LTE, the normal assumption is that the relay nodes are controlled, either completely or partially, by access nodes such as eNBs (also known as “mother eNBs”). When a relay node is powered on, it is required that an eNB has to be selected as a mother eNB before the relay node can become fully operational. This is because it is not yet connected to the core network side, and relaying a UE connection is feasible only through the mother eNB.
FIG. 2 shows a schematic diagram of an architecture of a relay-enhanced access network with a radio-relayed extension.
According to FIG. 2, it may be assumed that the radio access network RAN comprises one or more cells, each of which is served by one eNB as an access node or radio base station. The relay node RN is connected to its mother eNB through the SX interface (i.e. the relay link), and the eNBs are connected to the backbone/core network via the S1 interface, respectively. For load sharing/balancing and handover purposes, the eNBs communicate with each other through the X2 interface. Note that the association between the mother eNB and the RN can either be static or dynamic, i.e. The mother eNB role can be assigned to another eNB while the RN is active. As representative examples for backbone/core nodes of the backbone/core network, there are exemplarily depicted two MME/EPC (MME: mobility management entity, EPC: evolved packet core) gateways. Any eNB is connected with one MME/EPC gateway.
The architecture shown in FIG. 2 is a solution for enabling relaying in a LTE environment. However, in this architecture, there exist problems regarding system performance due to potential capacity bottlenecks.
The fact that there is a one-to-one correspondence between a relay node RN and an access node eNB (i.e. though an eNB can be connected to multiple RNs, a RN is connected only to one eNB), might limit the system performance because the end-to-end performance of relayed UEs will be constrained by the capacity available on the backbone link between the mother eNB and the core network (i.e. the link that is accessible through the S1 interface). For example, even if there are sufficient radio resources for the relay link, the performance of relayed UEs can degrade, if there is congestion in the backbone. In practice, S1 links are expensive, and there is usually not enough capacity on S1 links to support the maximum cell capacity offered by the air interface provided by an access node eNB.
Apart from the backbone that can turn out to be a capacity bottleneck, there may be insufficient resources on the relay link between the relay node RN and the mother eNB. In such a case, load sharing/balancing with a lightly loaded neighboring eNB (i.e. cell) could be conceivable. Such a scenario is illustrated in FIG. 3.
FIG. 3 shows a schematic diagram illustrating the variability of load distribution between two neighboring cells of a relay-enhanced access network.
As can be gathered from FIG. 3, there are not enough resources in the cell of the mother eNB (on the right hand side of FIG. 3) for the relay link, while the neighboring eNB (on the left hand side of FIG. 3) is very lightly loaded. Though handover to the lightly loaded cell may be an option for dealing with this load situation, a handover is not a totally flexible solution as back and forth handover of the relay and all its relayed UEs between eNBs can be an expensive procedure (e.g. in terms of capacity, overload, etc.). Moreover, with a handover, there may only be used the capacity of just one neighboring cell, instead of the sum of the available capacity in all the neighboring cells, which would be preferable. Relay nodes may also be multi-mode relay nodes, i.e. supporting multiple air interfaces. The use of multi-mode relay nodes may open the possibility of varying a load distribution between neighboring cells, where the relay node is connected at the same time to several eNBs via different interfaces, which can belong to different network technologies.
In this regard, it has been proposed that neighboring cells can communicate their load information via the X2 interface, which may then lead to a handover of some of the users to the neighboring cell, i.e. the load sharing is performed by handing over some of the users to the slightly loaded cell. However, after handover again, the same capacity bottlenecks as in the old cell (i.e. on the S1 interface and on the relay link) may prevail. Thus, performing a handover for load sharing may generally not improve the overall system performance in relay-enhanced access networks.
Accordingly, there does not exist any feasible solution for facilitating efficient load balancing in relay-enhanced access networks.